Compound Interest Calculator
Project the future value of an investment given a lump-sum principal, recurring monthly contributions, an annual rate, and a compounding frequency. Use it for long-horizon planning — retirement, college funds, or any goal where reinvested earnings drive the bulk of growth.
Last updated: May 2026
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About this calculator
Compound interest is the mechanism by which earnings themselves earn returns in later periods, producing exponential rather than linear growth. The calculator combines two components: (1) Lump-sum growth: Principal × (1 + r/n)^(n × y), where r is the annual rate as a decimal, n is compounding periods per year, and y is years; (2) The future value of recurring contributions added on top of the lump sum. Variables: Principal is the upfront balance; Monthly Contribution is added each month; Rate is annual APR; Compounds is periods per year (1=annual, 4=quarterly, 12=monthly, 365=daily, 52=weekly); Years is the holding period. Edge cases: a rate of 0% makes the contribution-formula's denominator zero — projections are unreliable at exactly zero rate; increasing compound frequency from 1 to 12 has noticeable effect (about 0.2% extra growth at typical rates), but from 12 to 365 is nearly invisible. The S&P 500's long-run nominal return of ~10% turns $10,000 into about $174,000 over 30 years compounded monthly — a 17× multiplier driven entirely by time. The biggest variable for retirement saving is not the rate but the time horizon and the consistency of contributions. Starting at 25 with $200/month vs starting at 35 with $400/month produces nearly the same end balance because the earlier saver gets 10 extra years of compounding on the early dollars.
How to use
Example 1 — Lump sum, no contributions. Principal $10,000, rate 7%, compounds 12 (monthly), years 20, monthly contribution $0. Step 1: (1 + 0.07/12)^(12 × 20) = 1.005833^240 ≈ 4.0387. Step 2: future value = 10,000 × 4.0387 ≈ $40,387. Verify ✓. The $10k roughly quadruples over 20 years — the textbook power of compounding when left undisturbed. Example 2 — Larger lump sum, longer horizon. Principal $25,000, rate 6%, compounds 365 (daily), years 25, monthly contribution $0. Step 1: r/n = 0.06/365 ≈ 0.0001644; n × y = 9,125. Step 2: (1.0001644)^9125 ≈ 4.482. Step 3: future value = 25,000 × 4.482 ≈ $112,050. Verify ✓. Note: switching from monthly (n=12) to daily (n=365) compounding adds only ~0.1% to the result — the headline rate matters far more than the frequency once you are above weekly compounding.
Frequently asked questions
What is the difference between compound interest and simple interest?
Simple interest pays the same dollar amount each period because it is calculated only on the original principal: $1,000 at 5% simple interest pays $50 every year forever. Compound interest pays interest on both the principal AND any accumulated interest, so the dollar payout grows each period: $1,000 at 5% compounded annually pays $50 the first year, $52.50 the second, $55.13 the third, and so on. Over short periods the difference is small; over 30 years it is enormous — $1,000 at 5% simple interest grows to $2,500 (linear), while compound grows to about $4,322 (exponential). Most banking and investing products use compounding rather than simple interest, which is why long-horizon savers benefit so much from starting early. Government bonds, payday loans, and some short-term commercial paper are the exceptions that still use simple-interest math.
How important is the compounding frequency (annual, monthly, daily)?
Less important than most people think once you are above annual compounding. The marginal effect drops off quickly: at 6% annual rate over 10 years, annual compounding grows $1,000 to $1,791, monthly to $1,819, daily to $1,822, and continuous to $1,822. The jump from annual to monthly adds about 1.5%; from monthly to daily adds 0.2%; from daily to continuous adds essentially nothing. This is why bank advertising touting "compounded daily" is mostly marketing — the underlying rate matters orders of magnitude more than the compounding period. The one place frequency does matter materially is for credit cards charging 20%+ APR with daily compounding, where the effective APR (APR + compounding effect) can run 22–24%.
What are the most common mistakes when using compound interest projections?
The biggest is using nominal returns without inflation adjustment. A 7% nominal return with 3% inflation is only 4% real — the projected future balance has half the purchasing power of the nominal number after 30 years. Always project in real terms if you want comparable purchasing power. The second is using equity-market average returns (10% nominal historical) for short or medium horizons where actual returns vary widely; any 5-year period might return −30% to +50%. The third is assuming constant compounding with no withdrawals — real-world investing involves taxes (drag of 1–2% per year in taxable accounts), fees (0.05–1.5% for funds), and behavioural drag (selling low and buying high costs investors 2–4% per year per Dalbar's research). The fourth is letting the compounding result anchor unrealistic expectations: a $10k investment becoming $1M in 30 years requires 16.6% annual returns, which is extraordinarily rare. Finally, many people use this calculator to justify procrastination — 'I'll start saving when I have more money' — when the math actually says start now even at small amounts.
When should I NOT use compound interest projections?
Avoid compound projections for time horizons shorter than 3–5 years; over short periods, the variance in actual returns dominates the smooth exponential curve and the projection is meaningless. Skip it for assets with non-compounding cash flows: bond coupons that you spend rather than reinvest, dividend stocks where you take the income, or annuities with fixed payouts. Do not use it for investments where the rate of return is not stable: individual stocks, real estate values, business investments, or commodity prices — these need scenario analysis or Monte Carlo simulation, not a single-rate projection. Skip it for tax-inefficient situations where the after-tax compounding rate is materially below the nominal rate — for high earners in taxable brokerage, the after-tax growth rate can be 25–35% lower than the pre-tax headline. Finally, do not use compound projections to compare investments with materially different risk profiles; the headline future value of a 12% projection looks great until you account for the 60% drawdown risk hidden inside it.
How long does it take to double my money at different rates?
The rule of 72 gives a quick mental shortcut: years to double ≈ 72 / annual rate. At 1% (savings account), money doubles in 72 years; at 4% (high-yield savings or short Treasuries), 18 years; at 6%, 12 years; at 8%, 9 years; at 10% (long-run S&P 500 average), 7.2 years; at 15% (aggressive growth target), about 5 years. The rule is most accurate between 5% and 15% — it slightly overstates time at very low rates and understates at very high rates. For high-precision work, use the actual formula: years = ln(2) / ln(1 + r) ≈ 0.693 / ln(1 + r). This rule is one of the most useful pieces of financial math because it lets you compare investment options instantly without a calculator: a 6% bond doubles in 12 years; an 8% stock fund doubles in 9. Over 30 years, the stock fund doubles 3.3 times (becomes ~10×) while the bond doubles 2.5 times (becomes ~5.7×) — that is the entire long-term case for equities over fixed income.